Network Working Group                                  C. Schmutzer, Ed.
Internet-Draft                                               C. Filsfils
Intended status: Informational                               Z. Ali, Ed.
Expires: 6 November 2022                                         F. Clad
                                                     Cisco Systems, Inc.
                                                           P. Maheshwari
                                                            Airtel India
                                                                R. Rokui
                                                                   Ciena
                                                                A. Stone
                                                                   Nokia
                                                                L. Jalil
                                                                 Verizon
                                                                 S. Peng
                                                     Huawei Technologies
                                                                 T. Saad
                                                        Juniper Networks
                                                                D. Voyer
                                                             Bell Canada
                                                              5 May 2022


                 Circuit Style Segment Routing Policies
                  draft-schmutzer-pce-cs-sr-policy-02

Abstract

   This document describes how Segment Routing (SR) policies can be used
   to satisfy the requirements for strict bandwidth guarantees, end-to-
   end recovery and persistent paths within a segment routing network.
   SR policies satisfying these requirements are called "circuit-style"
   SR policies (CS-SR policies).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."




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   This Internet-Draft will expire on 6 November 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Reference Model . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  CS-SR Policy Characteristics  . . . . . . . . . . . . . . . .   5
   5.  CS-SR Policy Creation . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Maximum Segment Depth . . . . . . . . . . . . . . . . . .   7
   6.  Recovery Schemes  . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Unprotected . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  1:1 Protection  . . . . . . . . . . . . . . . . . . . . .   9
     6.3.  Restoration . . . . . . . . . . . . . . . . . . . . . . .  10
       6.3.1.  1+R Restoration . . . . . . . . . . . . . . . . . . .  10
       6.3.2.  1:1+R Restoration . . . . . . . . . . . . . . . . . .  10
   7.  Operations, Administration, and Maintenance (OAM) . . . . . .  11
     7.1.  Connectivity Verification . . . . . . . . . . . . . . . .  11
     7.2.  Performance Measurement . . . . . . . . . . . . . . . . .  11
     7.3.  Candidate Path Validity Verification  . . . . . . . . . .  12
   8.  External Commands . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Candidate Path Switchover . . . . . . . . . . . . . . . .  12
     8.2.  Candidate Path Recomputation  . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   12. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  13
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     13.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18






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1.  Introduction

   Segment routing does allow for a single network to carry both typical
   IP (connection-less) services and connection-oriented transport
   services commonly referred to as "private lines".  IP services
   typically require ECMP and TI-LFA, while transport services that
   normally are delivered via dedicated circuit-switched SONET/SDH or
   OTN networks do require:

   *  Persistent end-to-end traffic engineered paths that provide
      predictable and identical latency in both directions

   *  Strict bandwidth commitment per path to ensure no impact on the
      Service Level Agreement (SLA) due to changing network load from
      other services

   *  End-to-end protection (<50msec protection switching) and
      restoration mechanisms

   *  Monitoring and maintenance of path integrity

   *  Data plane remaining up while control plane is down

   Such a "transport centric" behavior is referred to as "circuit-style"
   in this document.

   This document describes how SR policies
   [I-D.ietf-spring-segment-routing-policy] and the use of adjacency-
   SIDs defined in the SR architecture [RFC8402] together with a
   stateful Path Computation Element (PCE) [RFC8231] can be used to
   satisfy those requirements.  It includes how end-to-end recovery and
   path integrity monitoring can be implemented.

   SR policies that satisfy those requirements are called "circuit-
   style" SR policies (CS-SR policies).

2.  Terminology

   *  BSID : Binding Segment Identifier

   *  CS-SR : Circuit-Style Segment Routing

   *  ID : Identifier

   *  LSP : Label Switched Path

   *  LSPA : LSP attributes




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   *  OAM : Operations, Administration and Maintenance

   *  OF : Objective Function

   *  PCE : Path Computation Element

   *  PCEP : Path Computation Element Communication Protocol

   *  PT : Protection Type

   *  SID : Segment Identifier

   *  SLA : Service Level Agreement

   *  SR : Segment Routing

   *  STAMP : Simple Two-Way Active Measurement Protocol

   *  TI-LFA : Topology Independent Loop Free Alternate

   *  TLV : Type Length Value

3.  Reference Model

   The reference model for CS-SR policies is following the Segment
   Routing Architecture [RFC8402] and SR Policy Architecture
   [I-D.ietf-spring-segment-routing-policy] and is depicted in Figure 1.

                         +--------------+
         +-------------->|     PCE      |<--------------+
         |               +--------------+               |
         |                                              |
         |                                              |
         v   <<<<<<<<<<<<<< CS-SR Policy >>>>>>>>>>>>>  v
   +-------+                                          +-------+
   |       |=========================================>|       |
   |   A   | SR-policy from A to Z                    |   Z   |
   |       |<=========================================|       |
   +-------+                    SR-policy from Z to A +-------+

             Figure 1: Circuit-style SR Policy Reference Model

   By nature of CS-SR policies, paths will be computed and maintained by
   a stateful PCE defined in [RFC8231].  The stateful PCE provides a
   consistent simple mechanism for initializing the co-routed
   bidirectional end to end paths, performing bandwidth allocation
   control, as well as monitoring facilities to ensure SLA compliance
   for the live of the CS-SR Policy.  When using a MPLS data plane



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   [RFC8660], PCEP extensions defined in [RFC8664] will be used.  When
   using a SRv6 data plane [RFC8754], PCEP extensions defined in
   [I-D.ietf-pce-segment-routing-ipv6] will be used.

   In order to satisfy the requirements of CS-SR policies, each link in
   the topology MUST have:

   *  An adjacency-SID which is:

      -  Manually allocated or persistent : to ensure that its value
         does not change after a node reload

      -  Non-protected : to avoid any local TI-LFA protection to happen
         upon interface/link failures

   *  The bandwidth available for CS-SR policies specified

   *  A per-hop behavior ([RFC3246] or [RFC2597]) that ensures that the
      specified bandwidth is available to CS-SR policies at all times
      independent of any other traffic

   When using a MPLS data plane [RFC8660] existing IGP extensions
   defined in [RFC8667] and [RFC8665] and BGP-LS defined in [RFC9085]
   can be used to distribute the topology information including those
   persistent and unprotected adjacency-SIDs.

   When using a SRv6 data plane [RFC8754] the IGP extensions defined in
   [I-D.ietf-lsr-isis-srv6-extensions] and
   [I-D.ietf-lsr-ospfv3-srv6-extensions] and BGP-LS extensions in
   [I-D.ietf-idr-bgpls-srv6-ext] apply.

4.  CS-SR Policy Characteristics

   A CS-SR policy has the following characteristics:

   *  Requested bandwidth : bandwidth to be reserved for the CS-SR
      policy

   *  Bidirectional co-routed : a CS-SR policy between A and Z is an
      association of an SR-Policy from A to Z and an SR-Policy from Z to
      A following the same path(s)

   *  Deterministic and persistent paths : segment lists with strict
      hops using unprotected adjacency-SIDs

   *  Not automatically recomputed or reoptimized : the SID list of a
      candidate path must not change automatically to a SID list
      representing a different path (for example upon topology change)



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   *  Multiple candidate paths in case of protection/restoration:

      -  Following the SR policy architecture, the highest preference
         valid path is carrying traffic

      -  Depending on the protection/restoration scheme (Section 6),
         lower priority candidate paths

         o  may be pre-computed

         o  may be pre-programmed

         o  may have to be disjoint

   *  Connectivity verification and performance measurement is activated
      on each candidate path (Section 7)

5.  CS-SR Policy Creation

   A CS-SR policy between A and Z is configured both on A (with Z as
   endpoint) and Z (with A as endpoint) as shown in Figure 1.

   Both nodes A and Z act as PCC and delegate path computation to the
   PCE using the extensions defined in [RFC8664].  The PCRpt message
   sent from the headends to the PCE contains the following parameters:

   *  BANDWIDTH object (Section 7.7 of [RFC5440]) : to indicate the
      requested bandwidth

   *  LSPA object (section 7.11 of [RFC5440]) : to indicate that no
      local protection requirements

      -  L flag set to 0 : no local protection

      -  E flag set to 1 : protection enforcement (section 5 of
         [I-D.ietf-pce-local-protection-enforcement])

   *  ASSOCIATION object ([RFC8697]) :

      -  Type : Double-sided Bidirectional with Reverse LSP Association
         ([I-D.ietf-pce-sr-bidir-path])

      -  Bidirectional Association Group TLV ([RFC9059]) :

         o  R flag is always set to 0 (forward path)

         o  C flag is always set to 1 (co-routed)




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   If the SR-policies are configured with more than one candidate path,
   a PCEP request is sent per candidate path.  Each PCEP request does
   include the "SR Policy Association" object (type 6) as defined in
   [I-D.ietf-pce-segment-routing-policy-cp] to make the PCE aware of the
   candidate path belonging to the same policy.

   The signaling extensions described in
   [I-D.sidor-pce-circuit-style-pcep-extensions] are used to ensure that

   *  Path determinism is achieved by the PCE only using segment lists
      representing a strict hop by hop path using unprotected adjacency-
      SIDs.

   *  Path persistency across node reloads in the network is achieved by
      the PCE only including manually configured adjacency-SIDs in its
      path computation response.

   *  Persistency across network changes is achieved by the PCE not
      performing periodic nor network event triggered re-optimization.

   Bandwidth adjustment can be requested after initial creation by
   signaling both requested and operational bandwidth in the BANDWIDTH
   object but the PCE is not allowed to respond with a changed path.

   As discussed in section 3.2 of [I-D.ietf-pce-multipath] it may be
   necessary to use load-balancing across multiple paths to satisfy the
   bandwidth requirement of a candidate path.  In such a case the PCE
   will notify the PCC to install multiple segment lists using the
   signaling procedures described in section 5.3 of
   [I-D.ietf-pce-multipath].

5.1.  Maximum Segment Depth

   A Segment Routed path defined by a segment list is constrained by
   maximum segment depth (MSD), which is the maximum number of segments
   a router can impose onto a packet.  [RFC8491], [RFC8476], [RFC8814]
   and [RFC8664] provide the necessary capabilities for a PCE to
   determine the MSD capability of a router.  The MSD constraint is
   typically resolved by leveraging a label stack reduction technique,
   such as using Node SIDs and/or BSIDs (SR architecture [RFC8402]) in a
   segment list, which represents one or many hops in a given path.

   As described in Section 4, adjacency-SIDs without local protection
   are to be used for CS-SR policies to ensure no ECMP, no rerouting due
   to topological changes nor localized protection is being invoked on
   the traffic, as the alternate path may not be providing the desired
   SLA.




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   If a CS-SR Policy path requires SID List reduction, a Node SID cannot
   be utilized as it is eligible for traffic rerouting following IGP re-
   convergence.  However, a BSID can be programmed to a transit node, if
   the following requirements are met:

   *  The BSID is unprotected, hence only has one candidate path

   *  The BSID follows the rerouting and optimization characteristics
      defined in Section 4 which implies the SID list of the candidate
      path MUST only use unprotected adjacency-SIDs.

   This ensures that any CS-SR policies in which the BSID provides
   transit for do not get rerouted due to topological changes or
   protected due to failures.  A BSID may be pre-programmed in the
   network or automatically injected in the network by a PCE.

6.  Recovery Schemes

   Various protection and restoration schemes can be implemented.  The
   terms "protection" and "restoration" are used with the same subtle
   distinctions outlined in section 1 of [RFC4872], [RFC4427] and
   [RFC3386] respectively.

   *  Protection : another candidate path is computed and fully
      established in the data plane and ready to carry traffic

   *  Restoration : a candidate path may be computed and may be
      partially established but is not ready to carry traffic

   The term "failure" is used to represent both "hard failures" such
   complete loss of connectivity detected by Section 7.1 or degradation,
   a packet loss ratio, beyond a configured acceptable threshold.

6.1.  Unprotected

   In the most basic scenario no protection nor restoration is required.
   The CS-SR policy has only one candidate path configured.  This
   candidate path is established, activated (O field in LSP object is
   set to 2) and is carrying traffic.

   In case of a failure the CS-SR policy will go down and traffic will
   not be recovered.

   Typically two CS-SR policies are deployed either within the same
   network with disjoint paths or in two completely separate networks
   and the overlay service is responsible for traffic recovery.





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6.2.  1:1 Protection

   For fast recovery against failures the CS-SR policy is configured
   with two candidate paths.  Both paths are established but only the
   candidate with higher preference is activated (O field in LSP object
   is set to 2) and is carrying traffic.  The candidate path with lower
   preference has its O field in LSP object set to 1.

   Appropriate routing of the protect path diverse from the working path
   can be requested from the PCE by using the "Disjointness Association"
   object (type 2) defined in [RFC8800] in the PCRpt messages.  The
   disjoint requirements are communicated in the "DISJOINTNESS-
   CONFIGURATION TLV"

   *  L bit set to 1 for link diversity

   *  N bit set to 1 for node diversity

   *  S bit set to 1 for SRLG diversity

   *  T bit set to enforce strict diversity

   The P bit may be set for first candidate path to allow for finding
   the best working path that does satisfy all constraints without
   considering diversity to the protect path.

   The "Objective Function (OF) TLV" as defined in section 5.3 of
   [RFC8800] may also be added to minimize the common shared resources.

   Upon a failure impacting the candidate path with higher preference
   carrying traffic, the candidate path with lower preference is
   activated immediately and traffic is now sent across it.

   Protection switching is bidirectional.  As described in Section 7.1,
   both headends will generate and receive their own loopback mode test
   packets, hence even a unidirectional failure will always be detected
   by both headends without protection switch coordination required.

   Two cases are to be considered when the failure impacting the
   candidate path with higher preference is cleared:

   *  Revertive switching : re-activate the candidate path, change O
      field from 0 to 2 and start sending traffic over it

   *  Non-revertive switching : do not activate the candidate path,
      change O field from 0 to 1, keep the second candidate path active
      with O field set to 2 and continue sending traffic over it




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6.3.  Restoration

6.3.1.  1+R Restoration

   Compared to 1:1 protection described in Section 6.2, this restoration
   scheme avoids pre-allocating protection bandwidth in steady state,
   while still being able to recover traffic flow in case of a network
   failure in a deterministic way (maintain required bandwidth
   commitment)

   The CS-SR policy is configured with two candidate paths.  The
   candidate path with higher preference is established, activated (O
   field in LSP object is set to 2) and is carrying traffic.

   The second candidate path with lower preference is only established
   and activated (O field in LSP object is set to 2) upon a failure
   impacting the first candidate path in order to send traffic over an
   alternate path through the network around the failure with
   potentially relaxed constraints but still satisfying the bandwidth
   commitment.

   The second candidate path is generally only requested from the PCE
   and activated after a failure, but may also be requested and pre-
   established during CS-SR policy creation with the downside of
   bandwidth being set aside ahead of time.

   As soon as failure(s) that brought the first candidate path down are
   cleared, the second candidate path is getting deactivated (O field in
   LSP object is set to 1) or torn down.  The first candidate path is
   activated (O field in LSP object is set to 2) and traffic sent across
   it.

   Restoration and reversion behavior is bidirectional.  As described in
   Section 7.1, both headends use connectivity verification in loopback
   mode and therefore even in case of unidirectional failures both
   headends will detect the failure or clearance of the failure and
   switch traffic away from the failed or to the recovered candidate
   path.

6.3.2.  1:1+R Restoration

   For further resiliency in case of multiple concurrent failures that
   could affect both candidate paths of 1:1 protection described in
   Section 6.2, a third candidate path with a preference lower than the
   other two candidate paths is added to the CS-SR policy.






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   The third candidate path enables restoration and will generally only
   be established, activated (O field in LSP object is set to 2) and
   carry traffic after failure(s) have impacted both the candidate path
   with highest and second highest preference.

   The third candidate path may also be requested and pre-computed
   already whenever either the first or second candidate path went down
   due to a failure with the downside of bandwidth being set aside ahead
   of time.

   As soon as failure(s) that brought either the first or second
   candidate path down are cleared the third candidate path is getting
   deactivated (O field in LSP object is set to 1), the candidate path
   that recovered is activated (O field in LSP object is set to 2) and
   traffic sent across it.

   Again restoration and reversion behavior is bidirectional.  As
   described in Section 7.1, both headends use connectivity verification
   in loopback mode and therefore even in case of unidirectional
   failures both headends will detect the failure or clearance of the
   failure and switch traffic away from the failed or to the recovered
   candidate path.

7.  Operations, Administration, and Maintenance (OAM)

7.1.  Connectivity Verification

   The proper operation of each segment list is validated by both
   headends using STAMP in loopback measurement mode as described in
   section 4.2.3 of [I-D.ietf-spring-stamp-srpm].

   As the STAMP test packets are including both the segment list of the
   forward and reverse path, standard segment routing data plane
   operations will make those packets get switched along the forward
   path to the tailend and along the reverse path back to the headend.

   The headend forms the bidirectional SR Policy association using the
   procedure described in [I-D.ietf-pce-sr-bidir-path] and receives the
   information about the reverse segment list from the PCE as described
   in section 4.5 of [I-D.ietf-pce-multipath]

7.2.  Performance Measurement

   The same STAMP session is used to estimate round-trip loss as
   described in section 5 of [I-D.ietf-spring-stamp-srpm].






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   The same STAMP session used for connectivity verification can be used
   to measure delay.  As loopback mode is used only round-trip delay is
   measured and one-way has to be derived by dividing the round-trip
   delay by two.

7.3.  Candidate Path Validity Verification

   A stateful PCE is in sync with the network topology and the CS-SR
   Policies provisioned on the headend routers.  As described in
   Section 4 a path must not be automatically recomputed after or
   optimized for topology changes.  However there may be a requirement
   for a PCE to tear down a path if the path no longer satisfies the
   original requirements, detected by PCE, such as insufficient
   bandwidth, diversity constraint no longer met or latency constraint
   exceeded.

   The PCC may measure the actual bandwidth utilization of a CS-SR
   policy and report it to the PCE in order for the PCE to take an
   appropriate action if necessary.

   For a CS-SR policy configured with multiple candidate paths, a PCC
   may switch to another candidate path if the PCE decided to tear down
   the active candidate path.

8.  External Commands

8.1.  Candidate Path Switchover

   It is very common to allow operators to trigger a switch between
   candidate paths even if no failure is present.  I.e. to proactively
   drain a resource for maintenance purposes.  Operator triggered
   switching between candidate paths is unidirectional and has to be
   requested on both headends.

8.2.  Candidate Path Recomputation

   While no automatic re-optimization or pre-computation of CS-SR policy
   candidate paths is allowed as specified in Section 4, network
   operators trying to optimize network utilization may explicitly
   request a candidate path to be re-computed at a certain point in
   time.

9.  Security Considerations

   TO BE ADDED






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10.  IANA Considerations

   This document has no IANA actions.

11.  Acknowledgements

   The author's want to thank Samuel Sidor, Mike Koldychev, Rakesh
   Gandhi and Tarek Saad for providing their review comments.

12.  Contributors

   Contributors' Addresses

   Brent Foster
   Cisco Systems, Inc.
   Email: brfoster@cisco.com

   Bertrand Duvivier
   Cisco System, Inc.
   Email: bduvivie@cisco.com

   Stephane Litkowski
   Cisco Systems, Inc.
   Email: slitkows@cisco.com

   Jie Dong
   Huawei Technologies
   Email: jie.dong@huawei.com

13.  References

13.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

13.2.  Informative References

   [I-D.ietf-idr-bgpls-srv6-ext]
              Dawra, G., Filsfils, C., Talaulikar, K., Chen, M.,
              Bernier, D., and B. Decraene, "BGP Link State Extensions
              for SRv6", Work in Progress, Internet-Draft, draft-ietf-
              idr-bgpls-srv6-ext-09, 10 November 2021,
              <https://www.ietf.org/archive/id/draft-ietf-idr-bgpls-
              srv6-ext-09.txt>.




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   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extensions to Support Segment Routing over
              IPv6 Dataplane", Work in Progress, Internet-Draft, draft-
              ietf-lsr-isis-srv6-extensions-18, 20 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-lsr-isis-srv6-
              extensions-18.txt>.

   [I-D.ietf-lsr-ospfv3-srv6-extensions]
              Li, Z., Hu, Z., Cheng, D., Talaulikar, K., and P. Psenak,
              "OSPFv3 Extensions for SRv6", Work in Progress, Internet-
              Draft, draft-ietf-lsr-ospfv3-srv6-extensions-03, 19
              November 2021, <https://www.ietf.org/archive/id/draft-
              ietf-lsr-ospfv3-srv6-extensions-03.txt>.

   [I-D.ietf-pce-local-protection-enforcement]
              Stone, A., Aissaoui, M., Sidor, S., and S. Sivabalan,
              "Local Protection Enforcement in PCEP", Work in Progress,
              Internet-Draft, draft-ietf-pce-local-protection-
              enforcement-05, 4 May 2022,
              <https://www.ietf.org/archive/id/draft-ietf-pce-local-
              protection-enforcement-05.txt>.

   [I-D.ietf-pce-multipath]
              Koldychev, M., Sivabalan, S., Saad, T., Beeram, V. P.,
              Bidgoli, H., Yadav, B., Peng, S., and G. Mishra, "PCEP
              Extensions for Signaling Multipath Information", Work in
              Progress, Internet-Draft, draft-ietf-pce-multipath-05, 30
              March 2022, <https://www.ietf.org/archive/id/draft-ietf-
              pce-multipath-05.txt>.

   [I-D.ietf-pce-segment-routing-ipv6]
              Li, C., Negi, M., Sivabalan, S., Koldychev, M.,
              Kaladharan, P., and Y. Zhu, "PCEP Extensions for Segment
              Routing leveraging the IPv6 data plane", Work in Progress,
              Internet-Draft, draft-ietf-pce-segment-routing-ipv6-13, 1
              April 2022, <https://www.ietf.org/internet-drafts/draft-
              ietf-pce-segment-routing-ipv6-13.txt>.

   [I-D.ietf-pce-segment-routing-policy-cp]
              Koldychev, M., Sivabalan, S., Barth, C., Peng, S., and H.
              Bidgoli, "PCEP extension to support Segment Routing Policy
              Candidate Paths", Work in Progress, Internet-Draft, draft-
              ietf-pce-segment-routing-policy-cp-07, 21 April 2022,
              <https://www.ietf.org/archive/id/draft-ietf-pce-segment-
              routing-policy-cp-07.txt>.





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   [I-D.ietf-pce-sr-bidir-path]
              Li, C., Chen, M., Cheng, W., Gandhi, R., and Q. Xiong,
              "Path Computation Element Communication Protocol (PCEP)
              Extensions for Associated Bidirectional Segment Routing
              (SR) Paths", Work in Progress, Internet-Draft, draft-ietf-
              pce-sr-bidir-path-09, 6 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-pce-sr-bidir-
              path-09.txt>.

   [I-D.ietf-spring-segment-routing-policy]
              Filsfils, C., Talaulikar, K., Voyer, D., Bogdanov, A., and
              P. Mattes, "Segment Routing Policy Architecture", Work in
              Progress, Internet-Draft, draft-ietf-spring-segment-
              routing-policy-22, 22 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              segment-routing-policy-22.txt>.

   [I-D.ietf-spring-stamp-srpm]
              Gandhi, R., Filsfils, C., Voyer, D., Chen, M., Janssens,
              B., and R. Foote, "Performance Measurement Using Simple
              TWAMP (STAMP) for Segment Routing Networks", Work in
              Progress, Internet-Draft, draft-ietf-spring-stamp-srpm-03,
              1 February 2022, <https://www.ietf.org/archive/id/draft-
              ietf-spring-stamp-srpm-03.txt>.

   [I-D.sidor-pce-circuit-style-pcep-extensions]
              Sidor, S., Ali, Z., and P. Maheshwari, "PCEP extensions
              for Circuit Style Policies", Work in Progress, Internet-
              Draft, draft-sidor-pce-circuit-style-pcep-extensions-00, 7
              March 2022, <https://www.ietf.org/archive/id/draft-sidor-
              pce-circuit-style-pcep-extensions-00.txt>.

   [RFC1925]  Callon, R., "The Twelve Networking Truths", RFC 1925,
              DOI 10.17487/RFC1925, April 1996,
              <https://www.rfc-editor.org/info/rfc1925>.

   [RFC2597]  Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
              "Assured Forwarding PHB Group", RFC 2597,
              DOI 10.17487/RFC2597, June 1999,
              <https://www.rfc-editor.org/info/rfc2597>.

   [RFC3246]  Davie, B., Charny, A., Bennet, J.C.R., Benson, K., Le
              Boudec, J.Y., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,
              <https://www.rfc-editor.org/info/rfc3246>.





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   [RFC3386]  Lai, W., Ed. and D. McDysan, Ed., "Network Hierarchy and
              Multilayer Survivability", RFC 3386, DOI 10.17487/RFC3386,
              November 2002, <https://www.rfc-editor.org/info/rfc3386>.

   [RFC4427]  Mannie, E., Ed. and D. Papadimitriou, Ed., "Recovery
              (Protection and Restoration) Terminology for Generalized
              Multi-Protocol Label Switching (GMPLS)", RFC 4427,
              DOI 10.17487/RFC4427, March 2006,
              <https://www.rfc-editor.org/info/rfc4427>.

   [RFC4872]  Lang, J.P., Ed., Rekhter, Y., Ed., and D. Papadimitriou,
              Ed., "RSVP-TE Extensions in Support of End-to-End
              Generalized Multi-Protocol Label Switching (GMPLS)
              Recovery", RFC 4872, DOI 10.17487/RFC4872, May 2007,
              <https://www.rfc-editor.org/info/rfc4872>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8476]  Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
              "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
              DOI 10.17487/RFC8476, December 2018,
              <https://www.rfc-editor.org/info/rfc8476>.

   [RFC8491]  Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
              "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
              DOI 10.17487/RFC8491, November 2018,
              <https://www.rfc-editor.org/info/rfc8491>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.




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   [RFC8664]  Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
              and J. Hardwick, "Path Computation Element Communication
              Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
              DOI 10.17487/RFC8664, December 2019,
              <https://www.rfc-editor.org/info/rfc8664>.

   [RFC8665]  Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
              H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
              Extensions for Segment Routing", RFC 8665,
              DOI 10.17487/RFC8665, December 2019,
              <https://www.rfc-editor.org/info/rfc8665>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

   [RFC8697]  Minei, I., Crabbe, E., Sivabalan, S., Ananthakrishnan, H.,
              Dhody, D., and Y. Tanaka, "Path Computation Element
              Communication Protocol (PCEP) Extensions for Establishing
              Relationships between Sets of Label Switched Paths
              (LSPs)", RFC 8697, DOI 10.17487/RFC8697, January 2020,
              <https://www.rfc-editor.org/info/rfc8697>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

   [RFC8800]  Litkowski, S., Sivabalan, S., Barth, C., and M. Negi,
              "Path Computation Element Communication Protocol (PCEP)
              Extension for Label Switched Path (LSP) Diversity
              Constraint Signaling", RFC 8800, DOI 10.17487/RFC8800,
              July 2020, <https://www.rfc-editor.org/info/rfc8800>.

   [RFC8814]  Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
              and N. Triantafillis, "Signaling Maximum SID Depth (MSD)
              Using the Border Gateway Protocol - Link State", RFC 8814,
              DOI 10.17487/RFC8814, August 2020,
              <https://www.rfc-editor.org/info/rfc8814>.

   [RFC9059]  Gandhi, R., Ed., Barth, C., and B. Wen, "Path Computation
              Element Communication Protocol (PCEP) Extensions for
              Associated Bidirectional Label Switched Paths (LSPs)",
              RFC 9059, DOI 10.17487/RFC9059, June 2021,
              <https://www.rfc-editor.org/info/rfc9059>.




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   [RFC9085]  Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
              H., and M. Chen, "Border Gateway Protocol - Link State
              (BGP-LS) Extensions for Segment Routing", RFC 9085,
              DOI 10.17487/RFC9085, August 2021,
              <https://www.rfc-editor.org/info/rfc9085>.

Authors' Addresses

   Christian Schmutzer (editor)
   Cisco Systems, Inc.
   Email: cschmutz@cisco.com


   Clarence Filsfils
   Cisco Systems, Inc.
   Email: cfilsfil@cisco.com


   Zafar Ali (editor)
   Cisco Systems, Inc.
   Email: zali@cisco.com


   Francois Clad
   Cisco Systems, Inc.
   Email: fclad@cisco.com


   Praveen Maheshwari
   Airtel India
   Email: Praveen.Maheshwari@airtel.com


   Reza Rokui
   Ciena
   Email: rrokui@ciena.com


   Andrew Stone
   Nokia
   Email: andrew.stone@nokia.com


   Luay Jalil
   Verizon
   Email: luay.jalil@verizon.com





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   Shuping Peng
   Huawei Technologies
   Email: pengshuping@huawei.com


   Tarek Saad
   Juniper Networks
   Email: tsaad@juniper.net


   Daniel Voyer
   Bell Canada
   Email: daniel.voyer@bell.ca






































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